The mechanisms underlying and influencing the intestinal adsorption of lipids will be studied using the laboratory rat as the animal model. The lipids of interest are fatty acids, cholesterol, and phospholipids and the plant sterols. These compounds interact with one another and with the bile salts in the intestinal lumen to form molecular aggregates or micelles of variables size and composition. Micelle formation is a key step in the luminal phase of intestinal lipid transport. The rate limiting step in the absorption of lipids is diffusion across a so-called unstirred water layer overlying the villi. Presumably the micelles exist to aid in overcoming this rate-limiting barrier, and the effectiveness with which they do so depends on their size and composition, and hence their diffusivity. It is this aspect of the problem that will be stressed in the hopes that an improved understanding of micellar interactions and their effects on the absorption of cholesterol and other lipids will lead to a better basis for their control in disease entities such as malabsorption syndromes and hypercholesterolemia.